The thymidine analogue 5-bromodeoxyuridine (BUdR) can be phosphorylated and incorporated into DNA of replicating cells, where it replaces thymine as bromo-uracil (BU). The consequences of this substitution consist of a blockage of terminal differentiation without interfering with cell division or with basic cell function. Cultured myoblasts will continue to divide in the presence of BUdR but will not fuse to form myotubes. Similar results have been obtained with other types of tissue when cultured in vitro. The primary goal of our experiments is to prevent differentiation of specific cell types, such as Purkinje cells, in the central nervous system. Since the birthdate and final division of various types of neurons is known, it will be easy to have the cells incorporate BUdR during their last division. In order to follow the cell products with accuracy, tritiated BUdR will be given. The localization of the cells formed from the labeled division can be easily determined and the morphology and ultrastructural differentiation will be examined with light and electron microscopy. In this manner it will be possible to investigate whether cell differentiation can be prevented and to examine if this causes motor and behavioral deficits.